1. Field of the Invention
This invention relates to a method for manufacturing a gear fit article by fitting a gear part into a support member and to the article produced thereby.
2. Description of the Prior Art
Generally, a gear fit article, such as, for example, a rotor of a rotary piston engine, consists of a rotor body and an internal force fitted gear. The structure of such a gear fit article is shown, for example, in U.S. Pat. No. 3,762,377.
The manufacturing process of a rotor for use in a rotary engine typically comprises forming a steel work in the form of the internal gear by hot forging, heat treating to increase the surface hardness of the steel work, using, for example, quenching and tempering techniques, machining and gear cutting the steel work to form the desired internal gear. Thereafter, the rotor body and the internal gear are drilled together to form the necessary holes for pins to hold the rotor body and internal gear in a coupled state. Next, the internal gear is force fitted into the rotor body, and the pins are inserted into the commonly drilled, registering holes of the internal gear and the rotor body to fasten them together.
However, the gear fit article produced according to this known manufacturing process lacks fatigue strength due to the gear surface of the internal gear not having sufficient hardness.
Although nitriding treatments are known as a hard facing method for increasing fatigue strength of steel, steel treated by nitriding cannot be machined by gear cutting and drilling because the treated nitrided steel surface is very hard. Also, drilling a hole in a nitrided steel surface can be accomplished only with decreased accuracy because the steel surface is deformed by the heating utilized during the nitriding treatment.
Japanese Unexamined patent publication No. 52-128847 (1977) discloses treating a formed article to be nitrided with a metal plating to cover areas not to be nitrided to prevent the hard facing, and then nitriding the remainder of the article. This method is difficult to apply to a gear because of the requirement to plate all of the gear except the gear teeth which need the nitriding. This difficulty results in a serious impact on productivity.
While a rotary piston engine is operated, the fasteners which fix the internal gear to the rotor body receive stress due to the turning effort. Spring pins are normally used as the fasteners for absorbing this stress. However, these spring pins usually do not have sufficient strength to withstand large shearing forces.
The internal gear is typically force fit to the rotor body. However, the machined surfaces of the internal gear and the rotor body which engage due to the force fit do not have sufficient surface smoothness to create a high sticking force between the internal gear and the rotor body. Consequently, the shearing force imposed on the spring pins remains high and cannot be appropriately decreased by the sticking force between the internal gear and the rotor body.